# Copyright 2013, Michael H. Goldwasser
#
# Developed for use with the book:
#
#    Data Structures and Algorithms in Python
#    Michael T. Goodrich, Roberto Tamassia, and Michael H. Goldwasser
#    John Wiley & Sons, 2013
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

from ..exceptions import Empty


class LinkedQueue(object):
	"""FIFO queue implementation using a singly linked list for storage."""

	# -------------------------- nested _Node class --------------------------
	class _Node(object):
		"""Lightweight, nonpublic class for storing a singly linked node."""
		__slots__ = '_element', '_next'  # streamline memory usage

		def __init__(self, element, next):
			self._element = element
			self._next = next

	# ------------------------------- queue methods -------------------------------
	def __init__(self):
		"""Create an empty queue."""
		self._head = None
		self._tail = None
		self._size = 0  # number of queue elements

	def __len__(self):
		"""Return the number of elements in the queue."""
		return self._size

	def is_empty(self):
		"""Return True if the queue is empty."""
		return self._size == 0

	def first(self):
		"""Return (but do not remove) the element at the front of the queue.

		Raise Empty exception if the queue is empty.
		"""
		if self.is_empty():
			raise Empty('Queue is empty')
		return self._head._element  # front aligned with head of list

	def dequeue(self):
		"""Remove and return the first element of the queue (i.e., FIFO).

		Raise Empty exception if the queue is empty.
		"""
		if self.is_empty():
			raise Empty('Queue is empty')
		answer = self._head._element
		self._head = self._head._next
		self._size -= 1
		if self.is_empty():  # special case as queue is empty
			self._tail = None  # removed head had been the tail
		return answer

	def enqueue(self, e):
		"""Add an element to the back of queue."""
		newest = self._Node(e, None)  # node will be new tail node
		if self.is_empty():
			self._head = newest  # special case: previously empty
		else:
			self._tail._next = newest
		self._tail = newest  # update reference to tail node
		self._size += 1





